People Who Say Item Sets Are Not Useful
So some people, and apparently the developers at Riot, are like, "Mooo, item sets sucks and is for noob retard bronze guy. Fuck your item sets, good players make-up situational item sets on the fly for every game."
Ok.... Well let's look at if that is really true or not.
How long did it take Machine Gun build Kog'maw to become commonly used? It was like almost the whole season! People didn't just stumble ass backwards into that build on the fly, someone had to actually sit down and think about the different item synergies and how they synergized with Kog's various abilities, and realize that Wit's End (!) was a key ingredient to the mix, and gradually once people figured it out then the idea spread and other people COPIED the build those guys came up with and... NOTHING FUCKING HAPPENED ON THE FLY!
Same thing with Korean build Jhin, how long it take before Duskblade of Draktharr started being widely used on Jhin and Miss Fortune and a lot of guys like that? A long fucking time, shit didn't happen on the fly, some guys figured it out by spending time and experimenting WITH DIFFERENT PLANNED BUILDS, NOT ON THE FLY, and then other people copied it when it was successful.
The fucking whole meta is that same way, some people in one way or another PLAN OUT ITEM SETS/BUILDS for different champions, tinker and experiment with them, and figure out what fucking works the best. THEN a lot of other people copy what was figured out for them already by someone else and maybe they tell themselves they are creating it on the fly, but that's not really what happened. If you build Youmuu, Draktharr, and Black Cleaver on Miss Fortune and think that is just some shit you figured out situationally in the middle of a game, you're kidding yourself.
IN OTHER WORDS, the whole meta comes from nothing but people planning and playing around with item sets. Being able to create custom item sets to remember what you built, what you want to try out building in the future, etc, is a core functionality of the game. And getting rid of the custom item set functionality won't change any of that, it will just make it much more tedious and inconvenient for many players to plan out and experiment with different plans and ideas for champion builds when they want to.